1. Field of the Invention
This invention generally relates to a microstrip antenna device having three frequencies which can be used in three frequency bands.
2. Description of the Prior Art
Generally, microstrip antennas comprise a dielectric sheet with a conductor mounted on one surface and a ground conductor mounted on the other surface. Such an antenna utilizes the radiation loss of an open planar resonance circuit. Attention is now being focused on such microstrip antennas because of their low profile, reduced weight, compactness and ease of manufacture. However, the frequency band of such antennas is generally narrow thereby limiting such antennas usefulness to a single specific frequency band.
Until recently, attention has been focused on communications using a single frequency band. For example, in the case of communications between a vehicle moving within a town or city and a communication station, the ability to utilize more than two frequency bands is desired to accurately send information in a minimal amount of time. Further, it is preferred to be able to use at least three frequency bands for controlling and/or monitoring the communication.
When a plurality of frequency bands are used in the same area, a minimal deviation between bands of 5% is preferred to minimize interference. Accordingly, a microstrip antenna having more than one frequency band is desirable because of the constraints on the band width.
A microstrip antenna having two resonance frequencies is disclosed in Japanese Laid-Open Patent No. 56-141605 (1981). This antenna has a radiating conductive element and a feeder point located along one of the midlines of the angles of intersection between a long and short axis thereof. In this antenna, the excitation can occur in a long axis mode or a short axis mode so that the antenna is usable over two frequency bands. While this may represent an improvement over single frequency band microstrip antennas, it is not capable of being used with three frequency bands.